During processing of silver halide photographic elements, the developed silver is oxidized to a silver salt by a suitable bleaching agent. The oxidized silver is then removed from the element in a fixing step.
The most common bleaching solutions contain complexes of ferric ion and various organic ligands. One primary desire in this industry is to design bleaching compositions which are more compatible with the environment. Thus it is desirable to reduce or avoid the use of ferric complexes as bleaching agents.
Peracid bleaching solutions, such as those containing peroxide, persulfate, perborate, perphosphate, perhalogen, percarboxylic acid or percarbonate bleaching agents, offer an alternative to the ferric complex bleaching solutions. They are less expensive and present lower chemical and biological demands on the environment since their by-products can be less harmful.
While persulfate bleaching agents have low environmental impact, they have the disadvantage that their bleaching activity is slow and thus require the presence of a bleaching accelerator. The most common bleaching accelerators are thiols that have offensive odors.
Because hydrogen peroxide reacts and decomposes to form water, a hydrogen peroxide based bleaching solution offers many environmental advantages over persulfate and ferric complex bleaching solutions. As a result, many publications describe hydrogen peroxide bleaching solutions, including U.S. Pat. No. 4,277,556 (Koboshi et al), U.S. Pat. No. 4,301,236 (Idota et al), U.S. Pat. No. 4,454,224 (Brien et al), U.S. Pat. No. 4,717,649 (Hall et al) and WO-A-92/01972 (published Feb. 6, 1992).
In addition, WO-A-92/07300 (published Apr. 30, 1992) and EP 0 428 101A1 (published May 22, 1991) describe hydrogen peroxide compositions for bleaching high chloride emulsions (that is, silver halide emulsions having more than 90 mol % silver chloride). These compositions comprise from 0.004 to 0.4 mole of chloride ions per liter of solution and have a pH in the range of 5 to 11. These particular bleaching solutions comprising lower amounts of chloride ion, however, cause vesiculation in the processed elements. Those having higher amounts of chloride ion, e.g., 0.4 mol/l fail to bleach effectively in short bleaching times.
Despite all of the efforts of researchers in the art, no hydrogen peroxide bleaching composition has been commercialized because of various problems including vesiculation (that is, blistering from evolution of oxygen), poor bleaching efficiency and solution instability.
There remains a need for commercially viable hydrogen peroxide bleaching solutions that are stable and nonvesiculating. Moreover, it would be useful to have such solutions for bleaching photographic elements having predominantly chloride silver halide emulsions.